The present invention relates to a tuner for use in high-frequency circuit devices such as television receivers, videotape recorders, and personal computers, and in particular to an Up/Down tuner (double conversion tuner) for use in CATV (cable television) receivers.
High-frequency circuit devices, such as television receivers, videotape recorders, and personal computers, vary in form according to the use to which they are put; among these, CATV (cable television) receivers use an Up/Down tuner. By first increasing the frequency of an inputted RF (radio frequency) signal in an up-converter, and then decreasing its frequency in a down-converter, an Up/Down tuner eliminates interference, etc. caused by contaminating external radio waves.
As shown in FIG. 17, a conventional Up/Down tuner is provided with an antenna terminal RF-IN, a first band-pass filter B.P.F.-1, a PIN attenuator PIN-ATT, an RF amplifier RF-AMP, an up-converter 50, a second band-pass filter B.P.F.-2, a down-converter 60, and an analog signal band-pass filter B.P.F.-A.
The up-converter 50 includes an up-converter mixing circuit U-MIX, a first PLL (Phase Locked Loop) 51, and a first local oscillator circuit LO-1, and the down-converter 60 includes a second PLL 61, a second local oscillator circuit LO-2, and a down-converter mixing circuit D-MIX.
A signal sent to the foregoing conventional Up/Down tuner is received by the antenna terminal RF-IN, and is sent through the first band-pass filter B.P.F.-1, the PIN attenuator PIN-ATT, and the RF amplifier RF-AMP, and inputted into the up-converter mixing circuit U-MIX.
The first PLL 51 controls the first local oscillator circuit LO-1 in accordance with the reception channel data, and inputs a first local oscillator signal into the up-converter mixing circuit U-MIX. In the up-converter mixing circuit U-MIX, the RF signal and the first local oscillator signal are mixed and converted into an IF (Intermediate Frequency) signal.
The foregoing IF signal is sent through the second band-pass filter B.P.F.-2 and inputted into the down-converter mixing circuit D-MIX of the down-converter 60.
The second PLL 61 controls the second local oscillator circuit LO-2 in accordance with the reception channel data, and inputs a second local oscillator signal into the down-converter mixing circuit D-MIX. In the down-converter mixing circuit D-MIX, the IF signal and the second local oscillator signal are mixed and converted into a second IF signal. The foregoing second IF signal is outputted through the analog signal band-pass filter B.P.F.-A.
Here, an automatic gain control (AGC) signal is inputted into the PIN attenuator PIN-ATT so that image contrast remains constant even if the strength of the received signal fluctuates.
FIGS. 18(a) through 18(e) are projection drawings of an Up/Down tuner with the foregoing structure, and FIG. 19 is a perspective drawing thereof. In the foregoing Up/Down tuner, as shown in FIGS. 18(a) through 18(e) and 19, in a single chassis, the foregoing members are divided from one another by partitions, which prevent interference due to noise produced by the respective blocks. Further, although not shown in FIGS. 18(a) through 18(e) and 19, upper and lower surfaces of the foregoing Up/Down tuner are provided with shield covers, which prevent the entry of external noise.
However, with the foregoing conventional structure, because of the existence of the first and second local oscillator signals, the IF signal, etc., and of high frequencies of n times these signals, it is insufficient merely to provide each of the foregoing blocks with partitions. In other words, in order to suppress spurious, cross-modulation, and other kinds of interference, which arise due to mutual influence of the foregoing frequencies, it is necessary to separate the first and second local oscillator signals from each other by as great a distance as possible. A problem with this is that it tends to increase the size of the Up/Down tuner.
Moreover, when the foregoing Up/Down tuner is used for digital applications, the wide band of the reception range makes the frequencies of the first and second local oscillator signals and the IF signal wide-band and high-frequency. In such a case, the higher the frequency of these signals, the more likely they are to travel, not only through the circuit, but also through the chassis. For this reason, there are limits to the effectiveness of merely separating the locations of the first and second local oscillator circuits, and it becomes difficult to suppress interference due to their mutual influence.
It is an object of the present invention to provide a high-quality Up/Down tuner which is able to reduce the mutual influence of up-converter and down-converter local oscillator circuits and the entry of external noise, without increasing the size of the Up/Down tuner.
In order to attain the foregoing object, an Up/Down tuner according to the present invention includes two or more frequency conversion sections contained in separate respective chassis, and a signal transmitting section linking the respective chassis containing the respective frequency conversion sections.
In the foregoing Up/Down tuner, each frequency conversion section is contained in a different respective chassis. For this reason, in comparison with a conventional Up/Down converter, in which a plurality of frequency conversion sections are contained in a single chassis, a signal produced by a local oscillator circuit in one frequency conversion section is less likely to travel through the chassis and influence the other frequency conversion section.
Further, in the Up/Down tuner according to the present invention, the signal transmitting section has a coaxial shielding structure, in which a grounded conductive material forms a shield surrounding a signal line which serves as the signal transmission path between the respective frequency conversion sections.
The foregoing structure can reduce unnecessary radiation, such as leakage of a signal from the local oscillator circuit, etc., of the frequency conversion section, as well as entry of external noise from the environment in which the tuner is placed.
Additional objects, features, and strengths of the present invention will be made clear by the description below. Further, the advantages of the present invention will be evident from the following explanation in reference to the drawings.